Add fields_to_exclude to start_tile_feature()

[ljbade]

Closes #77

@springmeyer Is this what you had in mind?

Needs a test case.

[springmeyer]

looks good for testing your optimization. we should also consider the alternative design of accepting a list of fields to include. I'd need to mull on this more to have a strong opinion.

while I'm brainstorming: The other thing we should add is filtering support such that you could pass down a 'mapnik::expression' string to be evaluated and throw out features not matching. the goal of this would be to allow rudimentary filtering support for datasource other than Postgres.

[ljbade]

@springmeyer I will have to read up on Mapnik expressions.

[ljbade]

@springmeyer not sure why the build is failing with the missing std::end std::begin on Travis. Compiles fine on my machine.

[springmeyer]

This pull has merit, but before being ready would need 1) unit tests, 2) gut check to make sure performance is not impacted (and if it is, some quicker method to skip the std::find if not needed).

[flippmoke]

Closing this pull request because it is so far behind master.